Vortex flow measuring devices are frequently applied for measuring the flow of fluids in pipelines, especially gas flows or vapor flows in a high temperature range. In the case of such vortex flow measuring devices, a flow impediment is arranged in the flow path in such a manner that the fluid can flow past on both sides of the flow impediment. In such a case, vortices form on both sides of the flow impediment. Over a broad range of Reynolds numbers, the vortices, in such a case, form alternately on either side of the flow impediment so that a staggered arrangement of vortices arises. This staggered arrangement of vortices is referred to as a Kármán vortex street. In such vortex flow measuring devices, the principle utilized is that the vortex shedding frequency, with which these vortices are formed, is proportional to the flow velocity of the respective fluid over a broad range of Reynolds numbers. Accordingly, the flow velocity can be determined from the registered vortex shedding frequency of the vortices, which is referred to as vortex frequency in the following, and a characteristic calibration factor for the respective type of vortex flow measuring device.
As a rule, a vortex flow measuring device has, in such a case, a measuring tube, in whose flow path a bluff body is arranged as a flow impediment. In such a case, the bluff body extends preferably in a diametral direction completely or over a considerable part of the inner cross section of the measuring tube in such a manner that the particular fluid can flow past both sides of the bluff body. As a rule in such a case, the bluff body has at least two shedding edges which, in given cases, can also be rounded, on both sides. The shedding edges support a shedding of vortices. Operationally, the measuring tube is applied in a pipeline whose fluid flow is to be measured so that the fluid flows through the measuring tube and at least partially against the bluff body.
Additionally, the vortex flow measuring device, as a rule, comprises at least one vortex sensor, which responds to pressure fluctuations produced by vortices. The vortex sensor is arranged downstream from the two shedding edges. In such case, the vortex sensor can be arranged within the bluff body or downstream from the bluff body, especially as a separate component. The pressure fluctuations registered by the vortex sensor are transduced to an electrical measurement signal, whose frequency is directly proportional to the flow velocity of the fluid. Supplementally, if the density of the fluid is ascertained or known, then the mass flow of the fluid can be calculated from the flow velocity and the density.
Vortex flow measuring devices of the type described are applied, above all, for measuring single phase media, especially fluids (liquids, gases), for example, a flow of steam, or a liquid flow. In special applications, however, it can happen that a second or yet other phases occur within a fluid flow. For the sake of simplicity, the first phase and the second phase of a two or more phase medium flowing in the pipeline is discussed in the following, wherein the first and second phases represent the two main phases with the largest mass flow fractions. Other phases can especially be contained in one or both phases, especially as solid particles. The first and the second phases of the flowing two or more phase medium can each be, in such a case, different aggregate states of the same material, as is the case, for example, in the case of a water rivulet in steam, or also be two different materials, such as, for example, sand entrained in a liquid, etc. Preferably, both the first as well as the second phase are, in each case, a fluid (liquid, gas). The droplet/particle flow can, in such a case, in turn, comprise more than only one substance, especially two different materials. Each of the further developments explained below, even if not explicitly noted each time (by the statement of “at least a second phase”), pertains to this variant. The invention is especially applicable to two phase mixtures, in which the density difference between the two phases is high in such a manner and the two phases do not or only slightly mix, so that the second phase in entrained in the form of particles or droplets by the flow of the first phase.
It is known that, in vortex flow measuring devices, the occurrence of two or more phases leads to errors in the flow velocity ascertained from the vortex frequency.
Fundamentally, there are different ways in which at least a second phase can be carried in a flow of a first phase, such as a gas flow. The at least a second phase can be carried in the flow especially as particles and/or droplets relatively uniformly distributed in the first phase. Additionally, the second phase can also flow as a wall flow, especially as a rivulet, along a tube wall of the relevant pipeline. Both of these types of flow of the second and also third phase can occur, depending on application, in parallel (at least 3 phases) or only individually (at least 2 phases).
An example of the occurrence of two different phases is the occurrence of liquid collections in gas lines. This case is especially relevant in the case of vapor lines (steam lines), in which water can form as the second phase. As mentioned above, the liquid collection in such a case can be carried as a distributed droplet flow in the first phase (here: gas), however, the liquid collection can also flow alternatively or supplementally as a wall flow along a tube or pipe wall of the relevant pipeline. Besides the previously mentioned flow forms, solid bodies, such as sand or larger particles can, for example, also be transported in liquid or gas flows in gas lines. In such a case, the entrained solid bodies, especially when these are finely grained, (mixed with a part of the first phase in given cases) flow as a wall flow along a tube or pipe wall of the respective pipeline. Alternatively or supplementally, the entrained solid bodies can be carried at least partially in the flow of the first phase as a particle flow, which is distributed relatively uniformly over the cross section of the tube.
In such case, it is desirable for many applications to detect the occurrence of a second phase in a flow of a first phase reliably and without essentially increased costs and, in given cases, to also determine the fraction of the second phase, especially its mass flow. This is especially the case in applications in which steam is transported over a long distance. Supplying hot steam in pipelines is utilized in industrial plants especially for providing energy, wherein for this, a high steam quality, which corresponds to a low fraction of liquid water, is required. Especially in such case, it is often required that the steam quality be greater than 95%. The steam quality is given, in such case, as the ratio of the mass flow of the steam fraction to the total mass flow composed of steam and condensed water. Hot steam, which is conveyed in pipelines, is also applied in the field of oil production.
The present invention primarily concerns the problem of providing reliable and near time monitoring and measuring of a particle and/or droplet flow of at least a second phase in a flow of a first phase, especially a gaseous first phase, flowing in a pipeline.
U.S. Pat. No. 4,674,337 describes an apparatus for determining the number and mass of particles, which are carried in a fluid flow of predetermined flow rate. In such case, the apparatus includes an essentially planar impingement area, which is arranged in the fluid flow in such a manner that a predetermined fraction of the particles impacts the impingement area, in each case, at essentially the same angle in such a manner that an accumulation of the particles on the impingement area is prevented. The particles produce, upon impact, acoustic signals, which are proportional to the kinetic energy of the particles. Additionally, the apparatus comprises means to lead the acoustic signals out of the flow as well as means to then transduce the acoustic signals to electrical signals. The electrical signals are then evaluated by electronics in such a manner that an estimate of the total number and mass of the individual particles are obtained therefrom. The detection of particles with an apparatus described in U.S. Pat. No. 4,674,337 requires, in such case, that a separate apparatus is provided, which must be inserted into or installed in a corresponding pipeline. This is associated with increased cost and effort.